Many people started using it to market their blogs
At times when social media wasn’t that powerful people depended on site’s such as Reddit and Digg to market their content. This gave those sites an extra push.
The topics discussed match the interests of young people
According to post on business insider most Reddit users are young people. The type of content present on Reddit , usually humor entertainment and news, appeals a lot to those people.
Reddit started before social media mattered much
In 2005 People needed to share things online and Reddit was one of the best options as social sites weren’t still that powerful nor loaded with features.
Content is mostly about pictures
Pictures are considered more interesting to many people than text. Many of Reddit’s posts are just interesting pictures that were found elsewhere on the internet.
The design is simple
While Reddit’s design is not modern still it’s very simple and helps people get the task done easily and efficiently.
Protected by the community
Reddit has managed to build a community in a time were its basic concept seemed much more original and appealing. The fact that the site has a very large community is protecting it from being copied.
In 1995 the design was considered good
Reddit started in 1995. During that period most websites were text based and people didn’t complain about the design.
Many recurring visitors
Many of Reddit’s visitors are hard core loyal fans. Those fans visit the site almost daily and result in the generation of massive recurring traffic. (See Study summary: How much users read on the web?)
Many people wonder how Reddit is popular despite its old text based interface. This kind of controversy led to more conversations about the site which in turn led to more popularity. (See Why controversy can make things popular?)
People care about functionality more than design
Some people argue that most people care about functionality more than they care about design and they cite Reddit as an example. Those people believe Reddit became popular because it helps users achieve their goals easily.
Digg , the strongest competitor died
Digg , the biggest competitor to Reddit, died. This left Reddit with almost no direct competition.
Some Sub-Reddits are very controversial
Some Sub-Reddits were very controversial to the extent that one of them , called r/jailbait, was mentioned in a report by CNN and had to be closed.
Why is Reddit so popular?
What Makes Reddit So Awesome? When did Reddit become really popular?